How to send HTTP chunked response to emulate a video stream using Proxygen and Folly?

Question

I am writing a HTTP video streaming server based on Facebooks Proxygen. There is no seeking planned. Using the proxygen::ResponseBuilder I am abled to send chunks of a webm encoded video as HTTP response, i.e. chunked transfer encoding is working. My problem is, that Proxygen waits for proxygen::ResponseBuilder::sendWithEOM() before it even sends the response headers. I would like it to actually send the data asap after each call to proxygen::ResponseBuilder::send().

I tried to run the ResponseBuilder calls from a lambda executed from the EventBaseThread using evb->runInLoop() and evb->runInEventBaseThread()

using namespace folly;
using namespace proxygen;

std::thread t([&](){
    EventBase* evb = EventBaseManager::get()->getExistingEventBase();    
    // send headers ...
    while ( chunks avail. ) {
        //...
        evb->runInLoop([&](){
            ResponseBuilder(downstream_)
                     .body(std::move(chunk))
                     .send();
        });
        //... 
    }
    // sendWithEOM ...
});
t.detach();

This code is called from the onRequest() method of my RequestHandler. I tried to call ResponseBuilder::send() without wrapping it into evb->runInLoop(), but Proxygen v0.25.0 with Folly v0.42.0 is prohibiting calls to ResponseBuilder::send() from another thread using an assert. I removed this assert from here: https://github.com/facebook/folly/blob/v0.42.0/folly/io/async/EventBase.cpp#L491.

Now the emulated streaming is working, but it's crashing, if there are parallel requests. I guess it was not meant to be used like this, that's what the assert is for. But maybe anyone knows how to use the Proxygen infrastructure properly for my usecase?

Answer 1

That's got the same issue. I got it working with something like this.

folly::EventBase* eventBase = folly::EventBaseManager::get()->getExistingEventBase();
thread t([&, eventBase]() {
    while( chunks exist ) {
        auto chunk = getChunk();    
        eventBase->runInEventBaseThread([&, chunk=chunk]() mutable {
            ResponseBuilder(downstream_).body(move(chunk)).send();
        });
    }
});
// sendWithEOM ...
t.detach();

Answer 2

using namespace folly;
using namespace proxygen;

//Get Event Base in worker thread and pass it to new thread. If you call this inside new thread then you won't get worker thread's event base.
EventBase* evb = EventBaseManager::get()->getExistingEventBase();   
std::thread t([&, evb](){

// send headers ...
while ( chunks avail. ) {
    //...
    evb->runInLoop([&](){
        ResponseBuilder(downstream_)
                 .body(std::move(chunk))
                 .send();
    });
    //... 
}
// sendWithEOM ...
});
t.detach();

So no need to comment assertion in EventBase source.